Integrins are heterodimeric cell surface adhesion receptors, each composed of an .alpha. and .beta. type I transmembrane subunit (Hynes, R. O., Cell 1992, 69, 11). Integrin .alpha..sub.IIb .beta..sub.3 is platelet-specific and plays a key role in hemostasis and thrombosis by binding to arginine-glycine-aspartic acid (RGD)-containing adhesive ligands (e.g., fibrinogen, yon Willebrand factor) (Shattil, S. J. et al., Curr. Opin. Cell Biol. 1994, 6, 695-704). A notable feature of this ligand-receptor interaction is its regulation by the cell. Platelets normally circulate in a "resting" state in which the apparent affinity of .alpha..sub.IIb .beta..sub.3 for ligands is low. However, the addition of a physiological agonist, such as thrombin or ADP, or of a direct activator of protein kinase C, such as phorbol myristate acetate (PMA), causes a rapid increase in receptor affinity, resulting in ligand binding and platelet aggregation (Bennett, J. S. and Vilaire, G., J. Clin. Invest. 1979, 64, 1393). This increase in affinity can be prevented or reversed by compounds that stimulate an increase in intracellular cyclic AMP, such as prostaglandin I.sub.2 (PGI.sub.2); or in cyclic GMP, such as nitric oxide (Grager, S. E. and Hawiger, J., J. Biol. Chem. 1982, 257, 14606; Nguyen, B. L. et al., Am. J. Physiol. Heart Circ. Physiol. 1991, 261, H1043). It is believed that the intracellular signals generated by agonists (or antagonists) trigger modifications in the cytoplasmic domains of the integrin subunits, thus leading to conformational changes in the extracellular portion of the receptor that increase (or decrease) accessibility of the ligand-binding site (Ginsberg, M. H. et al., Curt. Opin. Cell Biol. 1992, 4, 766).